1. Field of the Invention
Apparatuses consistent with the present invention relate to an illumination device used for a liquid crystal display (LCD), and more particularly, to an illumination device having a linear light source unit that is integrally formed with a flat light guide plate and emits linear light using point light sources such as light-emitting diodes (LEDs).
2. Description of the Related Art
Liquid crystal displays (LCDs) are widely used because of their light weight and low power consumption. Since LCDs, which are light receiving flat displays, cannot emit light by themselves, the LCDs use external light to produce an image. Accordingly, LCDs require illumination devices such as backlight units or front light units.
Illumination devices used for LCDs are classified according to the arrangement of light sources as either direct light or edge light types. Direct light types are configured such that a plurality of lamps are installed under a liquid crystal panel and directly emit light onto the liquid crystal panel. Edge light types are configured such that a lamp is installed at a side of a flat light guide plate and light emitted by the lamp is transmitted through the flat light guide plate to a liquid crystal panel.
Edge light type illumination devices may use linear light sources or point light sources. Cold cathode fluorescent lamps (CCFLs) are typical linear-light sources, and light-emitting diodes (LEDs) are typical point light sources. With recent progress in thin display technology, many illumination devices using thin, high-efficiency point light sources have been developed.
FIG. 1 is a cross-sectional view of a related art LCD. FIG. 2 is a perspective view of a related art backlight unit of the related art LCD of FIG. 1.
Referring to FIGS. 1 and 2, the related art LCD employs the related art backlight unit as an illumination device for emitting light to a liquid crystal panel 10. The backlight unit is disposed behind the liquid crystal panel 10. The backlight unit includes one or more LEDs 40, for example four LEDs, as point light sources, and a flat light guide plate 30 emitting light incident from the LEDs 40 to the liquid crystal panel 10. In detail, the four LEDs 40 are disposed along a side of the flat light guide plate 30 at predetermined intervals, and light emitted from the four LEDs 40 enters the light guide plate 30 through a light incident surface 31 of the flat light guide plate 30. The traveling path of the light entering the flat light guide plate 30 is changed by optical path changing means, for example, a dot print pattern 35, which is disposed on a bottom surface of the flat light guide plate 30, such that the light entering the flat light guide plate 30 is emitted through a light exit surface 33 of the flat light guide plate 30. The light emitted from the light exit surface 33 of the flat light guide plate 30 passes through a diffusion sheet 21, prism sheets 22 and 23, and/or a protector 24, and reaches the liquid crystal panel 10. Instead of the dot print pattern 35, a hologram pattern, a reverse prism pattern, or a reverse trapezoidal pattern may be used as the optical path changing means. A reflective plate 50 may be disposed behind the flat light guide plate 30 to reflect light from the back of the flat light guide plate 30.
FIG. 3A is a photograph illustrating luminance distribution on the light exit surface 33 of the related art backlight unit of FIG. 2. FIG. 3B is a graph illustrating luminance distribution measured along line A-A′ of FIGS. 2 and 3A.
Referring to FIGS. 3A and 3B, the related art backlight unit constructed as described above has a drawback in that the radiation angle of light emitted from the LEDs 40 is limited. Accordingly, although the plurality of LEDs 40 are used, there are regions where light beams emitted from the LEDs 40 overlap as well as regions where the light beams never reach. For this reason, referring to FIGS. 3A and 3B, the luminance distribution of light emitted from the light exit surface 33 of the flat light guide plate 30 is not uniform, and the irregularity is particularly noticeable near the light incident surface 31 of the flat light guide plate 30. That is, more dark and/or bright lines are found near the light incident surface 31 of the flat light guide plate 30.
The regions with the non-uniform luminance distribution in the flat light guide plate 30 of the related art backlight unit do not contribute to illumination of the liquid crystal panel 10.